To improve the fatigue limits of press-fitted parts of axles by sub-critical quenching, that is, water quenching from the temperature just below Ac1 point, the effects of the heat treatment conditions on microstructure, hardness, residual stress distribution, rotating bending fatigue limits for crack initiation, σw1, and that for failure, σw2 were examined. Steels used for the experiment were normalized 0.35-0.45 %C steels for machine structual uses. For fatigue testing, press-fitted specimens of 50mm in diameter were mainly used.
(1) The microstructure of specimen was hardly changed by sub-critical quenching, but the surface hardness was slightly increased.
(2) High residual compressive stress, distributed from the surface to considerable depth of the core, was detected in sub-critical quenching specimens.
(3) σw1 and σw2 of press-fitted specimens were both improved considerably by sub-critical quenching. Moreover, the limit depth of non-propagating crack, measured after the fatigue test in which applied stress was just below σw2, was also remarkably increased.
(4) It was supposed that σw1 of the press-fitted specimen could be improved by the increase of surface hardness, and σw2 by the increase of residual compressive stress in surface zone, respectively.